MnO2-Pt/C composite electrodes for preventing voltage reversal effects with polymer electrolyte membrane fuel cells

Water is produced at the cathode of proton-exchange membrane fuel cells (PEMFC). If water were not being removed effectively, it would accumulate at the cathode of PEMFC causing the electrode flooding. The consequence is oxygen starvation, thus increasing the concentration overpotential of the cathode. In the worst scenarios, a proton (H+) reduction reaction (PRR), instead of the oxygen reduction reaction (ORR), might occur at the cathode. Not only will this cause a cathode potential drop, but the output voltage of a single cell would likely be reversed due to oxygen starvation. This phenomenon is termed the voltage reversal effect (VRE) in this paper. To study and resolve the VRE problem, a MnO2-Pt/C composite electrode was used to replace the conventional Pt/C electrode. The authors suggest that the electrochemical reduction of MnO2 in the composite electrode has almost the same Nernstian potential as the ORR, which would serve as a substitute for the ORR in the case of oxygen starvation. Thus, the voltage reversal effect caused by the PRR could be avoided. Two environments, N-2- and O-2-saturated H2SO4, were adopted to simulate two cases, i.e., O-2 starvation and O-2 rich. It was found that MnO2-Pt/C can prevent the voltage reversal effect to a certain extent. In a N-2-saturated 1 M H2SO4 solution, the current density of the Pt/C electrode made of 0.6 mg Pt cm(-2) was close to 0, while for the MnO2-Pt/C composite electrode made of 0.4 mg Pt cm(-2) and 0.8 mg MnO2 cm(-2), it was as high as 10 mA cm(-2). Though the current generated on the MnO2-Pt/C composite electrode in the case of oxygen starvation is not as great as that in the case when oxygen rich, it might be high enough for some cases, such as powering a radio, hearing-aid and so like miniature devices. In an O-2-saturated 1 M H2SO4, the presence of MnO2 in a MnO2-Pt/C composite electrode primarily plays a catalytic role in the ORR. It enhances the catalytic behavior of Pt for the ORR. The impedance spectra of MnO2-Pt/C and Pt/C electrodes were carried out for the two gases in bubbled electrolyte, which further confirmed that MnO2 in the composite electrode does substitute for oxygen as an electron-acceptor in the case of oxygen starvation. The discharged MnO2 can then be restored to its initial state, regardless of whether it is in oxygen rich or starved conditions. (c) 2006 Elsevier B.V. All rights. reserved.